Conventional key telephone systems detect incoming intermittent ringing signals, analyze an intermittent pattern of the ringing signals with a microprocessor of a main unit, select the closest one of the analyzed pattern among prototype patterns stored in the microprocessor, and then generate a ringing tone in synchronization with the prototype pattern thus selected.
Therefore, in the conventional key telephone systems, processes executed by the microprocessor are complicated due to that analysis of the intermittent pattern of the ringing signals; a required time to analyze the intermittent pattern of the ringing signals causes a delay in beginning the ringing tone; and it is required to store a variety of prototype patterns in order to correctly simulate the incoming ringing signals.